Method of controlling the rate of feed to an extraneously energized airswept mill

ABSTRACT

IN THE CONTROLLING OF THE RATE OF FEED OF FRESH MATERIAL TO AN EXTRANEOUSLY ENERGIZED AIRSWEPT MILL FROM WHICH ONLY PART OF THE OUTPUT OF MATERIAL IS EXTRACTED BY THE AIR STREAM AND A FURTHER PART DEPOSITED FROM THE MILL IS REFED MECHANICALLY TO THE MILL, A CONTROL SIGNAL IS DERIVED FROM THE DEGREE OF FULLNESS OF THE MILL, AND A CONTROL SIGNAL IS DERIVED FROM THE AMOUNT OF MATERIAL MECHANICALLY REFED TO THE MILL. ONE OF THE TWO CONTROL SIGNALS IS USED TO CONTROL THE NOMINAL OR TARGET VALUE OF THE RATE OF FEED OF FRESH MATERIAL, AND THE OTHER CONTROL SIGNAL IS USED TO CONTROL THE ACTUAL VALUE OF SUCH RATE OF FEED.

United States Patent Inventors Heinrich Henne Ennlgerloh; PeterTiggesbaumker, Oelde, Germany Appl. No. 790,595 Filed Jan. 13, 1969Patented June 28, 1971 Assignee PolysiusG.m.b.H.

Neubeckum, Germany Priority Feb. 28, 1968 Germany METHOD OF CONTROLLINGTHE RATE OF FEED TO AN EXTRANEOUSLY ENERGIZED AIRSWEPT MILL PrimaryExaminer- Donald G. Kelly Attorney-Marshall and Yeasting ABSTRACT: Inthe controlling of the rate of feed of fresh material to an extraneouslyenergized airswept mill from which only part ofthe output of material isextracted by the air stream and a further part deposited from the millis refed 4 claims 1 Drawing mechanically to the mill, a control signalis derived from the US. Cl 241/19, degree of fullness of the mill, and acontrol signal is derived 241/30, 241/34 from the amount of materialmechanically refed to the mill. Int. Cl ..B02c 25/00, One of the twocontrol signals is used to control the nominal B02c 21/00 or targetvalue of the rate of feed of fresh material, and the Field of Search241/19, 24, other control signal is used to control the actual value ofsuch 34,52,53,97 rate offeed.

ll ire-' r 1 1 r13 1 l l I I 1 l l l i 1 l I l 7 1 l l l I l l I I 2 6 lI l l 5 I a 3 l I 9- I METHOD OF CONTROLLING THE RATE OF FEED TO ANEXTRANEOUSLY ENERGIZED AIRSWEPT MILL BACKGROUND OF THE INVENTION Thisinvention relates to a method of controlling the rate of feed to anextraneously energized airswept mill, wherein only part of thecirculating material is extracted from the mill in the air stream, whilea further part of this material deposited from the mill is refed to themill by mechanical conveyor means.

The term extraneously energized mill is used herein to mean any grindingmill other than an autogeneous mill containing no grinding elements inwhich the material tumbles and shatters itself.

It is known to operate extraneously energized mills purely as airsweptmills. The entire quantity of ground material is then extractedpneumatically in the air stream..With such mills the feed rate issatisfactorily controlled according to the pressure differential acrossthe mill or some other parameter characteristic of the degree to whichthe mill is filled.

A disadvantage of this type of mill lies, however, in the rela tivelyhigh specific energy consumption of the exclusively pneumatic extractionof the ground material. In order to reduce energy consumption,extraneously energized mills have therefore been developed wherein onlypart of the circulating material is extracted from the mill in the airstream, while a further part of the material deposited from the mill isrefed to the mill by mechanical conveyor means. Since such mechanicalconveyance requires less energy than pneumatic conveyance, a reductionin specific energy consumption can be achieved in this manner.

These extraneously energized mills with partly mechanical and partlypneumatic extraction of material have also previously been controlled byparameters characteristic of the degree to which the mill is filled, inparticular by the pressure differential in the air stream passingthrough the mill. It is found, however, that in general optimum outputcannot be produced by such controls. A further disadvantage is that control by pressure differential alone does not respond quickly enough tochanges in the grindability, moisture content, grain size and otherproperties of material freshly supplied to the mill. Extraneouslyenergized mills operated by the known method therefore tend to run toofull. Since considerable time and labor is involved in stopping andemptying the mill after it runs too full, mills with partly mechanicaland partly pneumatic extraction of material, which are controlledaccording to the pressure differential only, are not operated up to theload limit of the mill. Consequently the mill is not working at itsoptimum operating point.

SUMMARY OF THE INVENTION The object of the invention is to provide amethod which avoids these disadvantages, permitting optimum control ofthe rate of feed to an extraneously energized mill with partly pneumaticderived partly mechanical extraction of material, and in particularpermitting the mill to function at its most favorable working point,i.e. its load limit.

According to the invention this object is achieved in that controlsignals are derived from both the degree of fullness in the mill and theamount of mechanically conveyed circulating material, one signal beingused to affect the nominal value and the other the actual value of thefeed rate adjustment.

Two basic variations of this method are possible:

Either the control signal derived from the degree of fullness in themill can be allowed to affect the nominal value, and the control signalderived from the amount of mechanically conveyed circulating materialthe actual value of the feed rate adjustment, or vice versa.

Which of these two variations is more desirable in a given case dependson the way the grinding plant is operated. If the greater part of thecirculating material is extracted in the air stream and only arelatively small proportion is fed back to the mill by a mechanicalconveyor, in general it is desirable to utilize the control signalderived from the degree of fullness in the mill as the primarycontrolsignal and act on the actual value of the feed rate control withit. When the control signal derived from the degree of fullness in themill is thus used as the primary control signal, a nominal or targetvalue for this control signal is established. Then whenever this controlsignal indicates that the mill is too full, the rate of feed of freshmaterial is reduced until this control signal again equals the nominalor target value. This nominal or target value, however, is varied inaccordance with the amount of material which is being mechanicallyrecirculated. Thus when the rate of flow of the mechanicallyrecirculated material is increased, the nominal or target value of theprimary control signal is changed so that the mill will be maintainedless full. The conditions are, however, reversed if the greater part ofthe circulating material is mechanically conveyed and only a residue-ispneumatically conveyed. In that case, the signal derived from the rateof flow of mechanically recirculated material will be used as thecontrol signal, in order to change the rate of feed of fresh materialwhenever the rate of flow of mechanically recirculated material deviatesfrom a nominal or target value. When the signal derived from the rate offlow of mechanically recirculated material is thus used as the primarycontrol signal, the nominal or target value for this primary controlsignal is varied in accordance with the signal derived from the degreeof fullness of the mill. Which of these systems is applied depends onthe grinding technique aspects in view of the requirements for dryingthe ground material.

In obtaining a control signal derived from the degree of fullness in themill, one may use:

the pressure differential across the mill, the quantity or speed of theair conveyed at a particular point in the conduit system connected tothe mill, the pressure differential across a cyclone separator, thepressure differential across an air current sieve or another part of theair feed system. ln obtaining a control signal derived from the amountof mechanically conveyed circulating material, one may use:

the power consumption of the bucket conveyor or other conveyor means formechanically feeding the circulating material,

the weight of circulating material as determined by a balance,

the power consumption of a separately powered sieve which may bedisposed between the bucket conveyor and the mill inlet.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a diagram of oneembodiment of plant controlled by the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The material to be ground isextracted from a hopper l by a variable-rate feeder 2 and fed through anair seal 3 to an extraneously energized mill 4. The finished materialand part of the circulating material are conveyed by an air stream to asieve 5. The finished material passes with the air stream into a cycloneseparator 6 where it is separated (arrow 8). The waste air is fed by afan 7 to a filter for purification. The coarse material is fed back tothe mill 4 from sieve 5. Part of the circulating material is extractedfrom the mill (pipe 10) and fed back to mill 4 via bucket conveyor 9 andair seal 3.

The feed rate of the feeder 2 is adjusted by a controller 11 thatreceives a control signal 12 which is derived from the degree offullness in mill 4, preferably from the pressure differential across themill, and which affects the actual value of the control. The nominalvalue of the control is acted on by a second control signal 13 derivedfrom the quantity of mechanically conveyed circulating material,preferably from the power consumption of the bucket conveyor 9.

As an additional safeguard, the nominal value of the degree of fullnessof the mill is given upper and lower limiting values,

in view of the above explanation the mode of operation of the controlmethod should readily be understood. If for instance the amount ofmaterial dropping from the mill 4 and fed back to it by bucket conveyor9 is increased by a change in grindability of the material, the controlsignal 13 fed to the controller 1 1 reduces the nominal or target valueof the signal derived from the degree of fullness of the mill. if theactual value of the control signal 12 remains steady at the instant whenthe nominal or target value of the control signal is reduced, the feedrate for fresh material is nevertheless reduced, because the nominal ortarget value for the primary control signal 12 has been changed.

With the embodiment described, all of the circulating material fallingfrom mill 4 is fed back to it by bucket conveyor 9 directly, i.e.without intermediate sifting. Sifting of the dropped and mechanicallyconveyed fraction of the circulating material is thus deliberatelyomitted. Extensive tests have in fact shown that the dropped andmechanically conveyed fraction of the ground material contains such asmall amount of finished material that the intermediate siftingpreviously thought necessary and desirable would not pay. The omissionof sifting the mechanically conveyed circulating material simplifies theplant and its operation considerably, and in particu' lar avoidsconstant maintenance of these parts of the plant which are subject toparticularly heavy wear in known plants.

Before being fed back into the mill the ground material falling from itpreferably only has any iron pieces separated out. This avoids thedanger of such iron pieces being fed back into the grinding process andcausing damage.

lclaim:

l. A method of controlling the rate, of feed of fresh material to anextraneously energized airswept mill from which only part of the outputof material is'extracted by the air stream, a further part depositedfrom the mill being refed mechanically to the mill, wherein theimprovement comprises the steps of deriving a control signal from thedegree offullness ofthe mill, deriving a control signal from the amountof material mechanically refed to the mill, using one of the controlsignals to control the rate of feed of fresh material, and using theother control signal to control the target value of the first controlsignal.

2. A method according to claim 1 wherein the target value of the firstcontrol signal is controlled-by the other control signal between anupper and a lower limiting value.

3. A method according to claim 1 wherein all of the material depositedfrom the mill is refed to the mill directly, without intermediatesifting.

4. A method according to claim 1 wherein iron pieces are removed fromthe deposited material before it is refed to the mill.

